A key Government adviser on poverty has been revealed as an ultra-rich "non-dom" who is able to avoid paying thousands of pounds a month in tax.

Multi-millionaire Jennifer Moses has taken out a mortgage on her £10million North London mansion using an offshore bank based in the Isle of Man.

The tactic allows the U.S.-born former investment banker to maintain her non-domiciled status and benefit from enormous tax breaks.

It is thought that she and her husband Ron Beller - a hedge fund manager - could save tens of thousands of pounds a year in tax by exploiting the loophole, although there is no suggestion they have done anything illegal.

Miss Moses, 45, joined Number 10 last month as head of special projects, developing policies designed to alleviate poverty by improving social mobility.

Critics of the appointment have argued that the mother of three might find it difficult to empathise with the poor.

She is a former managing director at the famously high-paying investment bank Goldman Sachs. Her husband is estimated to be worth £20million.

Such is Miss Moses's wealth that her former personal secretary, Joyti De-Laurey, was able to steal £1.2million from her without her noticing.

At the secretary's trial, it was revealed that Miss Moses spent £500,000 on her 40th birthday party and that her husband spent £17,000 on wine in a single year.

The court also heard how the couple's four-storey Hampstead home is so large that they frequently email each other rather than shout down the banisters.

Miss Moses and her husband bought the mansion when they settled in London in 1999, having lived in New York, Tokyo and Hong Kong.

In 2004, they renewed their mortgage on it, taking out a loan with Barclays Private Clients International, based in the Isle of Man. Using an offshore bank means the couple can remain legally "non-domiciled" in the UK even if they live here permanently.

And by keeping their money outside the UK, non-doms can escape paying income and capital gains tax. In Miss Moses's case that could amount to a huge amount of money every year.

Yesterday, politicians from Labour and opposition parties lined up to criticise the tactic from someone so close to the Prime Minister.

Liberal Democrat Treasury spokesman Lord Oakeshott said: "Millions trapped in Britain's mortgage chaos will see Number 10 as totally out of touch when Brown's adviser uses an offshore tax haven for her mansion in Hampstead.

"Tax avoidance by the rich picks the pockets of the poor, even when it's entirely legal."

Labour MP Peter Kilfoyle, a former Defence Minister, said: "At a time when the 10p starting tax rate is being chopped it appears that these non-doms are getting away with daylight robbery.

"It undermines the Government's credibility when the person taking advantage of these rules is themselves a Number 10 adviser."

Conservative MP David Davies said: "It smacks of rank hypocrisy that while Gordon Brown takes money hand over fist from British taxpayers, he is quite happy to have as a special adviser someone who is able to avoid tax with an offshore mortgage."

In February, Chancellor Alistair Darling said he planned to charge a £30,000 annual levy on non-doms, but watered down the proposal after critics warned it would provoke an exodus of talent and wealth.

In last month's Budget, Mr Darling did close the loophole by which non-doms were able to obtain offshore mortgages on UK properties.

However, deals already in place, such as that of Miss Moses, are unaffected. Yesterday a spokesman for Miss Moses - who is a director of Prince Charles's not-for-profit company Duchy Originals - said: "She is a dual U.S.-UK citizen who for historic reasons has nondomiciled status.

"She nevertheless pays tax in both jurisdictions in line with both the law and the tax code of both countries."

The spokesman was unable to say at which rate Miss Moses paid tax in the UK.

A Number 10 spokesman refused to comment on Miss Moses's case, but said: "All advisers abide by the relevant code of conduct."